(i) Field of the Invention
The present invention relates to a syndiotactic polypropylene copolymer. More specifically, the present invention relates to a copolymer having high syndiotacticity of propylene and another olefin.
The present invention further relates to novel extruded polypropylene articles. Specifically, the present invention is concerned with extruded articles, especially films and sheets and blown films, of a high-syndiotacticity propylene homopolymer or a high-syndiotacticity copolymer of propylene and another olefin.
(ii) Description of the Prior Art
A syndiotactic polypropylene has been known for a long time. Such a conventional polypropylene can be prepared by a low temperature polymerization in the presence of a conventional catalyst comprising a vanadium compound, an ether and an organic aluminum, but this kind of polypropylene is poor in syndiotacticity and scarcely exhibits characteristics of syndiotactic polypropylene. In addition, a copolymer of an .alpha.-olefin and propylene is hardly considered to be a crystalline polypropylene. On the other hand, a polypropylene having good tacticity, i.e., a syndiotactic pentad fraction of more than 0.7 as measured by .sup.13 C-NMR has been discovered for the first time by J. A. Ewen et al. which can be obtained by polymerizing propylene in the presence of a catalyst comprising a transition metal (Hf and Zr) compound having an asymmetric ligand and aluminoxane (J. Am. Chem. Soc., 110, 6255-6256, 1988).
The above-mentioned method which has been suggested by J. A. Ewen et al. exhibits high catalytic activity per unit amount of the transition metal, and can provide a polymer having a high tacticity and relatively properly balanced physical properties, but molded articles of this kind of polymer are inconveniently poor in transparency and unsatisfactory in impact resistance.
Isotactic polypropylene has a wide range of utility as molded or otherwise formed articles. As a major application field, it can be used for the production of extruded articles such as sheets and blown films. These sheets and blown films have relatively good physical properties but involve the problem that they are insufficient in transparency. There has hence been a long-standing demand for extruded polypropylene articles, especially sheets and blown films, having better transparency.